1. Field of the Invention:
The present invention relates to a method for image reproduction using an ion modulation electrode.
2. Description of the Prior Art:
An image reproducing device has been known as a device adapted to reproduce on a recording sheet image signals of information derived from a computer or transmitted through a communication line or, information which is obtained through converting an original image into electric signals by means of a solid state image sensor such as CCD.
Various types of the electrostatic devices have been proposed up to now. In electostatic reproduction, a latent image corresponding to the image signal is formed on a charge receptor, such as a photosensitive member, dielectric member or recording paper. As a means for forming the latent image, it has been proposed to use an ion modulation electrode which is adapted to modulate the corona ions produced by a corona wire or the like in accordance with the image signal, thereby to form a latent image corresponding to the image signal on a dielectric body.
FIG. 1 schematically shows an example of construction of the image reproducing device of the type described. This device has an illumination lamp 2 adapted to illuminate a document M mounted on a document glass plate 1. The light reflected by the document M is applied to a CCD 5 through a mirror 3 and a lens 4 to obtain an image signal corresponding to the document image. The image signal coming from the CCD 5 is applied after being amplified by an amplifier 6, to a signal processing circuit 7. The output information from the computer and information from a communication line are directly applied to the signal processing circuit 7. The image signal processed by the processing circuit 7 is applied to an ion modulation electrode driving circuit 8.
The ion modulation electrode 9 has an insulating layer sandwiched between a continuous layer 9a of a conductive material and segmented layer 9b of a conductive material, and is provided with a plurality of apertures 9c arranged in a row. An image signal corresponding to the image information to be reproduced is applied from the driving circuit 8 to the segmented layer 9b of conductive material, and is scanned by controlling the potential of segmented layer 9b by the potential of said layer 9a. A reference numeral 10 denotes a control circuit including a clock signal generator, which is adapted to apply the clock signal and a control signal to the CCD 5, signal processing circuit 7 and the driving circuit 8. Ion generator 11 is disposed above the ion modulation electrode 9, and a rotary drum 12 is disposed under the ion modulation electrode 9; the surface of rotary drum is covered a dielectric member. A developing device 13, transfer electrode 14, separating device 15, charge eliminating electrode 16 and a cleaning device 17 are disposed around the rotary drum 12 as in an ordinary electrophotographic process. A fixing device 18 is also provided as known per se. The ions generated by an ion generator 11 pass through the apertures 9c at a rate controlled by the control signal applied to the segmented layer 9b of conductive material of the ion modulation electrode 9, so that a latent image corresponding to the image information to be reproduced is formed on the dielectric member of the rotary drum 12. The electrostatic latent image thus formed is transferred to a recording paper P by means of the transfer electrode 14 on which the image has been developed by the developer by means of the developing device 13. The recording paper P is then separated from the rotary drum 12 by means of the separating device 15, and is ejected to the outside of the apparatus after the fixing of the toner image by the fixing device 18.
An image reproducing method using the ion modulation electrode produces an image of a high quality with a high resolution and good half tone reproduction, but suffers the disadvantage of low recording speed. This is attributable to the following two reasons: namely, (1) practical limit in current density of the corona ions (several tens of microamperes per square centimeter) and (2) a practical lower limit in ion modulating time due to ion mobility. In order to enhance the recording speed, it has been suggested to increase the density of the ion flow. This measure, however, requires a higher voltage for the ion generator, which in turn requires a power supply of higher voltage and increases the tendency of sparking discharge. To avoid this, it is necessary to employ a higher degree of electric insulation.
It has also been suggested to use an image signal holding technic which is generally adopted in the field of facsimile employing an electrostatic reproducing method using multi-stylus electrodes. In this method, the image signals received serially are not successively given to the multi-stylus electrodes but are temporarily stored and, after an accumulation of predetermined number of signals, applied to the multi-stylus electrodes. By so doing, it is possible to obtain a longer ion modulating time and to increase the recording speed.
However, in the formation of an electrostatic latent image by using the ion modulation electrodes, it takes a definite time for the ions produced by the ion generator to pass through the apertures of the ion modulation electrode, because the gap (20 to 100 .mu.m) between the continuous layer and segmented layer of the conductive material is greater than the discharge gap of the multi-stylus electrodes.
For this reason, there is a practical limit in the frequency of the image signal capable of reproducing the image. For instance, in the case of an ion modulation electrode of a thickness of 0.1 mm, the limit frequency is about 250 KHz under the electric field of 2.5 KV/cm, provided that the mobility of ion in the air is 2 cm/V sec. Thus, there is a practical upper limit of frequency of the image signal in the electrostatic reproducing using an ion modulation electrode.
A detailed description will be made hereinunder as to the upper limit of the frequency of the image signal.
The upper limit frequency is given by the following equation (1) EQU fmax=.mu..multidot.E/2d (1)
where, .mu. represents the mobility of ion in the air, E represents the strength of electric field in the ion modulation electrode, d represents the thickness of the ion modulation electrode including the continuous layer of conductive material, segmented layer of conductive material and the insulating layer sandwiched therebetween.
The mobility of the ion in the air is about 2 cm.sup.2 /V sec. Assuming that the strength E of the electric field is 2.5 KV/cm and that the thickness d is 0.1 mm, the upper limit frequency f max is calculated to be 250 KHz. Particularly, in the case where an ion modulation electrode is used, the upper limit of the frequency of the image signal is determined to be about 250 KHz. This upper limit of the frequency is a critical factor in obtaining higher recording speed.